Electromagnetic induction has been known for a long time and it has been used in many applications. In electromagnetic induction a time-varying magnetic flux induces an electromotive force to a closed conductor loop. Vice versa, a time-varying current creates a varying magnetic flux. In transformers, this phenomenon is utilized to transfer energy wirelessly from a circuit to another via inductively coupled coils. A primary coil transforms an alternating current into a varying magnetic flux, which is arranged to flow through the secondary coil. The varying magnetic flux then induces an alternating voltage over the secondary coil.
Wireless charging is an application where electromagnetic induction is used to transfer energy over air. A wireless charging system comprises a charger device i.e. a WLC transmitter with a primary coil, and a device to be charged, receiver device, i.e. a WLC receiver with a secondary coil. The current in the charger device is transferred to the receiver device through these electromagnetically coupled coils, and the induced current may be further processed and used to charge the battery of the charged device. Energy is transmitted through inductive coupling from the charger device to the charged device, which may use that energy to charge batteries or as direct power. The wireless charger device comprises a charging area, onto which the receiver device is arranged to receive inductive energy from the charger device. Other unwanted metallic foreign object(s) in that charging area during the inductive energy transmission may, however, heat up and cause safety hazards in addition to power losses.